In wireless communication networks, common topologies are a ‘star’ network operating in infrastructure mode and a ‘cluster’ network operating in ad hoc mode. In a star network, all nodes, e.g., mobile terminals, communicate packets indirectly with each other via a central node called a coordinator or access point (AP), e.g., a base station. The AP receives packets from transmitting nodes and forwards the packets to receiving nodes. In a cluster network, all terminals communicate packets directly with each other.
The operation of such networks can be according to the IEEE 802.15.3 and IEEE 802.15.4 standards, see “IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks: “Specific requirements Part 15.3: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for High Rate Wireless Personal Area Networks (WPANs),” 2003, and IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—“Specific requirements Part 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for High Rate Wireless Personal Area Networks (WPANs),” 2003.
Because the signals transmitted by all nodes share the same frequency band, it is necessary to enforce a channel access methodology in order to efficiently utilize the network bandwidth. This can be done with a channel access schedule, which determines when and how terminals can access the channel. The access schedule can be broadcast periodically using a beacon frame.
The beacon specifies network parameters, i.e., transmission rates, logical channels, network identifiers, and the channel access schedule. The period between successive beacon signals is called a frame. The beacon is followed by a contention period and a contention free period. The beacon frame defines the start of the contention period, the start of the contention free period, and the access schedule for the contention free period.
During the contention period, the terminals compete with each other to gain access to the physical channel. Typically, a random access method such as Aloha or CSMA is used. After gaining access, terminals transmit packets on the channel strictly according to the access schedule during the contention free period to guarantee interference free packet transmissions.
Recent advances in wireless communications, such as smart antennas, digital signal processing and VLSI, make it possible to provide very high-capacity wireless channels in the physical medium layer. These emerging physical-layer technologies offer at least an order-of-magnitude larger bandwidth than the bandwidths used by current generation standards.
For example, the IEEE 802.11n standard for a physical layer specification provides up to 100 Mb/s throughput at the medium access control (MAC) layer. The IEEE 802.15.3a standard for high-capacity wireless personal area networks aims at data rates of 110 Mb/s or higher using ultra-wideband (UWB) communications.
CSMA/CA Media Access Control
Wireless networks, based on the IEEE 802.11 standard, use a distributed coordination function (DCF) to control access to the physical channel. The DCF applies to both infrastructure and ad-hoc modes and can follow the well-known CSMA/CA medium access control (MAC).
Before each packet is transmitted, a node senses the channel, waits until the channel becomes idle, and then defers for a time interval called a DCF inter-frame space (DIFS). Then, the node enters a backoff stage and determines a random time interval called the backoff-time. The random time interval is uniformly distributed between zero and a size of a contention window (CW). After the backoff-time expires, an optional Request to Send/Clear to Send (RTS/CTS) is initiated between the two communicating nodes. After successful RTS/CTS, only one data packet is transmitted over the channel. If the packet is received correctly, the receiving node transmits an acknowledgement packet (ACK) for the single transmitted packet, unless the packet was broadcast to all nodes. If an ACK packet is not received, the packet is retransmitted, until it is correctly received.
To reduce the probability of collisions, the size of the CW is doubled after each perceived collision until a maximum value, CWmax, is reached. The size of the CW is reset to a fixed minimum value, CWmin, after a successful transmission of a packet to maintain channel efficiency.
The MAC for the IEEE 802.11e standard provides quality-of-service (QoS) for multiple contending nodes. The standard defines a hybrid coordination function (HCF), which combines DCF and point coordinated function (PCF) with enhanced QoS mechanisms. The contention based channel access mechanism in HCF is called an enhanced distributed channel access (EDCA). This provides fully distributed, differential channel access.
As shown in FIG. 1, a virtual collision handler 120 for EDCA uses four access categories (ACs) 100-103. Each of the ACs can be considered as an instance of the DCF described above. To achieve different channel access priorities, ACs are configured with different value of DIFS, now called arbitration IFS (AIFS) 110, CWmin and CWmax. Moreover, the access point can dynamically adjust these parameters by setting new values in the appropriate fields of the periodic beacons. Within the four ACs, AC0 and AC1 are typically employed for carrying best effort and background traffic at relatively low data rates, video streams use AC2, and audio streams use AC3 to attain a highest priority delivery at relatively high data rates.
Limitations of Current CSMA/CA MAC
The current 802.11 MAC is inefficient for a high-capacity physical layer. FIG. 2 illustrates the MAC-layer throughput achieved by DCF when the physical layer nominal bit rate is 216 Mb/s. As shown by curve 201, the current DCF MAC can only deliver about 48 Mb/s throughput when RTS/CTS is disabled, and curve 202 shows that a mere 30 Mb/s of throughput when the RTS/CTS is enabled. This is far below the target rate of 100 Mb/s at the MAC-layer, as specified by the IEEE 802.11n MAC standard.
The fundamental problem is that current MAC according to the IEEE 802.11 standard is rather inefficient, causing a significant reduction in bandwidth. The inefficiencies are due to excessive overhead for the RTS/CTS, packet preambles, acknowledgements, contention windows and various interframe-spacing parameters. The overhead becomes even more significant as the data rate increases. This is because the relative portion of channel time actually carrying data decreases, while the overhead remains constant. The packet preambles and various interframe spacing are fixed parameters determined by specific designs of the physical layer.
Hence, it is an object of the invention to reduce this overhead.
The current CSMA/CA based design cannot exploit the multi-rate capability to increase the overall channel utilization. The DCF MAC ensures roughly the same long-term access probability, hence throughput fairness, for each node, no matter the transmit rate used by the node. As a result, the channel is unnecessarily monopolized by low-rate nodes in terms of channel access time. The high-rate nodes only utilize a disproportionately lower amount of access time. This leads to sub-optimal overall channel throughput. As a byproduct, this also results in completely unfair time allocation for nodes with different transmission rates. The unfairness especially penalizes the throughput of the high-rate nodes and largely cancels out the high capacity offered at the physical layer.
Hence, it also an object of the invention to fully exploit multi-rate capabilities.
Communication-intensive applications, such as video and audio streams, are made possible by the new physical layer. These applications typically require quality-of-service (QoS) assurances in terms of minimum delay and/or minimum bandwidth, in order to function properly. However, current MAC solutions do not provide any performance bounds.
Hence, it also an object of the invention to support service differentiation among diverse applications to assure QoS statistically within a single MAC.